Sound-reducing housing for alternating current electric apparatus



Sheet /NVENTORZ THOMAS d'. TWOMEY,

rom/Ey T. J. TWOMEY SOUND-REDUCING HOUSING FOR ALTERNATING CURRENT ELECTRIC APPARATUS 1I I I I I FI I I 1.1 I I I I I I I L l I I I I V I I II n 4:8 I. Hv I I l I FL T f I I I I I I I I I 1 I I I I I I I I I I L v .--7 v I+ m u Q C |1I|| |I L IIr. .f, .V IIII I I I m F I I I l 31m J UII. IIIIII June 24,1969

Filed sept. 26, 1967 June 24, 1969 T. J. TwoMEY 3,451,503

SOUND-REDUCING HOUSING FOR ALTERNATING CURRENT ELECTRIC APPARATUS Filed sept. 26, 1967 l sheet /NVE/vro/Q: THOMAS J Two/MEV,

A TTOR/VEY June 24, 1969 1 1 TWOMEY 3,451,503

SOUND-REDUCING HOUSING FOR ALTERNATING CURRENT ELECTRIC APPARATUS Filed sept. 2e, '1967 sheet Q of 4 /NvE/v TOR: THOMAS J 7`woMY,

ATTORNEY June 24, 1969 r. J. TwoMEY SOUND-REDUCING HOUSING FOR ALTERNATING CURRENT ELECTRIC APPARATUS Filed sept. 26. 1967, sheet 1 of 4 /NvE/vron: THOMAS J. Two/15V g BY M ATTORNEY United States Patent O U.S. Cl. 181-33 6 Claims ABSTRACT OF THE DISCLOSURE In an air-cooled enclosure for electric transformers or the like, a ventilated housing spaced at its sides from the enclosed apparatus is provided with large air inlet and outlet openings near the top and the bottom of the housing. To prevent transmission of internally generated sound directly through the ventilated openings there is provided adjacent each opening a chambered air duct structure comprising a tubular air passageway having one Wall common with one or more laterally positioned sound attenuating cavities, each such cavity communicating with the air passageway through a tubular orice in the common wall and comprising with such orifice a Helmholtz resonator tuned to one selected sound frequency to be attenuated. Preferably the inner walls of the housing are also lined with sound-absorbing material .and the housing is mounted substantially in vibrationisolating independence of the enclosed electric apparatus.

BACKGROUND My invention relates to sound-reducing means for alternating current electric apparatus such as electric induction apparatus, land more particularly to improved sound eliminating housing structures of the ventilated type for air-cooled apparatus.

Power transformers which range in rating up to about 10,000 kva. at operating voltages up to about l5 kv. are frequently of the ventilated dry-type in order to render the equipment both fireproof and explosion proof. Because the housing for such transformers contains air, as distinguished from liquid, as an insulating fluid and coolant, it can be operated at relatively high temperatures. While their operating voltages are limited as a practical matter to about kv., such transformers increasingly constitute an important class of apparatus iinding wide-spread application inside residential and commercial buildings where ambient noise from other sources is at very low levels.

In ventilated dry-type transformers, the basic function of transforming voltage is performed by the usual core and coil assembly. In operation this assembly vibrates as a result of alternating current through the coil and emits sound comprised primarily of frequencies which are doubles of the fundamental and harmonic frequencies of the .applied voltage. For example, an applied voltage having a frequency of 6() cycles per second (c.p.s.) would result in sound frequencies of 120, 240, 360 c.p.s. etc., with the lower frequencies predominating in amplitude. Such transformers are usually provided with an enclosing housing of sheet metal to protect the high voltage portions of the core and coil assembly. To provide for forced or natural circulation of cooling air, such `a housing ordinarily has one opening near the bottom and at least one opening near the top. The housing provides inherently for some sound attenuation due to containment of the core and coil. However this containment principle, While normally resulting in a reduction in soundlevel outside the housing, actually increases the 3,451,503 Patented June 24, 1969 ICC sound level near the vicinity of the core and coil assembly because of rellection of sound energy Within the housing. Therefore the sound-containment effect is generally supplemented by sound-absorbing or attenuating means interposed in the air path between the core and housing and by providing vibration isolating supporting means between the housing base and the core and coil assembly. Vibration isolation is usually in the form of resilient mounts for the core and coil and flexible connections to housing mounted bushings. For attenuation of reflected sound in the air space between the core and housing, glass libre-lining or panels may be used to reduce, the internal build-up of sound by reection. These sound-reduction techniques `afford some improvement in external sound levels, but they are of limited effectiveness due to the relatively large inlet and outlet openings required for passage of cooling air. These openings are usually so large, especially on apparatus cooled by natural circulation of air, that direct transmission of sound therethrough is in itself objectionable.

SUMMARY Accordingly, it is a principal object of my invention to provide an improved sound attenuating housing for Ventilated dry-type electric induction apparatus, such as power tarnsformers and the like.

It is a more particular object of my invention to provide, in an air-cooled electric transformer having an enclosing housing with large air ilow apertures, means for reducing transmission of sound both through the housing and directly through the air flow apertures.

It is a specific object of my invention to provide, in a sou-nd-attenuating housing for air-cooled electric apparatus such `as power transformers, means for acoustically but not physically blocking the air flow openings in the housing thereby to supplement sound attenuating and vibration isolating means associated with the housing.

In carrying out my invention in one preferred embodiment, I provide a ventilated dry-type transformer having a core and coil assembly resiliently mounted in spaced relation appreciably above a supporting base and an enclosing housing independently mounted on the base to minimize direct transmission of vibration to the housing. The inner walls of the housing are preferably lined with glass fibre for attenuation of sound transmitted interiorly to the housing Walls. In juxtaposition with each large air flow opening through the housing, I provide a tubular air duct, such ducts preferably extending inwardly above and below the transformer core and coil assembly. Laterally adjacent to each such duct, and having a common wall therebetween, I provide at least one resonant cavity in communication with the duct through an orifice in the common wall. Each cavity and connecting orifice is tuned as a Helmholtz resonator to one selected sound frequency of predominant amplitude, thereby substantially to close the opening to transmission of sound at 'that frequency. Several resonant cavities or transversely extending sets of cavities tuned each to a ditferent sound frequency may be positioned in spaced relation along the length of the air duct.

DESCRIPTION My invention will be more fully understood and its various objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawings in which:

FIG. 1 is a front elevational view of a transformer housing including sound-reducing features embodying my invention, the transformer itself and certain internal housing elements being shown in phantom display by broken lines;

FIG. 2 is a vertical cross-sectional view of the trans- 3 former apparatus of FIG. 1 taken along the line A--A of FIG. 1;

FIG. 3 is a perspective view of an integral air duct and resonating chamber assembly included in the transformer housing shown at FIGS. 1 and 2; and

FIG. 4 is a graphical .representation of certain test results illustrating the relative degrees of attenuation of sound at various frequencies resulting from its passage through a resonant chambered duct such as that of FIG. 3.

Referring now to the drawings, and particularly to FIGS. l and 2, I have shown a ventilated dry-type transformer apparatus comprising a rectangular box-like housing 1 preferably formed of sheet metal and mounted directly upon a supporting pad or base 2. The base 2 may suitably be a rigid concrete foundation pad of relatively massive proportion. The front wall of the housing, as shown at FIG. 1, is provided at its lower end with a large air inlet aperture 3 extending across substantially the entire width of the housing, and the upper end of the opposite or rear wall is provided with a similar air outlet aperture 4 (shown at FIG. 1 in broken lines).

Within the housing 1 there is positioned a transformer core and coil assembly mounted upon the base or foundation 2 substantially in vibration-isolating independence of the housing 1. The transformer core illustrated comprises upper and lower yoke members 5 and 6, respectively, and three core legs 7, 8 and 9 extending in parallel spaced relation between the upper and lower yokes. Current-carrying coils or windings 10, 11 and 12 are positioned respectively on the core legs 7, 8 and 9. The upper yoke is provided with a pair of lifting brackets 14, 15 to which there may be connected suitable eye bolts (not shown) extending through the housing cover. The transformer core and coil assembly is mounted at an appreciable distance above the base 2 and upon two pairs of downwardly diverging supporting legs 20 and 21 fixed to opposite ends of the horizontal lower core yoke 6. Between the pairs of supporting legs 20, 21 and the base 2 there are interposed resilient bearing members 22, 23 of the character described in my Patent 3,312,290 issued on Apr. 4, 1967. It will of course be understood that while resilient support of the core and coil assembly enhances vibration isolation, a sufficiently massive foundation may in itself provide sufficient isolation.

The height of the supporting legs 20, 21 is ordinarily such that the spacing between the base 2 and the lower core yoke 6 is appreciable in regard to the height of the transformer core and coil assembly. As will be understood by those skilled in the art, some elevation of the core and coil assembly is necessary to provide for circulation of air beneath .the assembly, but the elevation is usually greater than required for this purpose in order to accommodate the height of the transformer to the height of adjacent switch gear ordinarily associated therewith. In any event, appreciable space is usually available beneath the core and between the pairs of supporting legs 20, 21, and it is into this space that the air inlet aperture 3 opens. The side walls of the housing 1 are also spaced an appreciable distance from the sides of the core and coil assembly in order to provide adequate circulation of air upwardly and around the sides of the core and coils. Similarly, ventilation and mechanical requirements direct that an appreciable space be provided between the top of the housing and the upper core yoke 5, and the air outlet opening 4 is adjacent this upper air space.

Within the housing 1, sound generated by vibration of the core and coil assembly is transmitted across the air passages to the side walls and top of the housing. This sound is ordinarily amplified by reflection back and forth between the housing and core and coil assembly. To attenuate sound transmitted within the housing in this manner, the inner walls of the housing are lined throughout substantially the entire inner surface with sheets 25 of sound absorbing material, preferably glass fibre or the like. As shown at FIG. 2, glass fibre sheets 25 are mounted in slightly spaced relation with respect to the inner walls of the housing, as on supporting studs 26. It will be understood by those skilled in the art that it is not essential that the glass fibre sheets 25 be positioned against or immediately adjacent housing Walls. They may, for example, be freely suspended at any point between the housing and the noise-generating source, i.e., the core and coil assembly.

The amount of air required for cooling, especially by natural circulation, is such that the air flow openings 3 and 4 in the lower and upper portions of the housing, respectively, are of considerable size. By natural circulation, air enters at the lower portion of the housing through the apertures 3 and is exhausted from the upper portion of the housing through the aperture 4, the path of circulation of the air upward through the housing being illustrated in broken lines at FIG. 2. The air ow openings 3 and 4 are necessarily of such area that unless some attenuating means is provided, appreciable sound would be transmitted directly through these openings from the interior to the exterior of .the housing. The vibration isolating mounting of the core and coil assembly and the sound-absorbing lining of the casing, while effective for the purposes intended, do not in themselves have any effect upon the -direct transmission of sound through the air flow apertures. It is desirable, if possible, to provide means for closing, or substantially closing, these apertures to the transmision of sound without physically blocking or constricting the ow of -air therethrough, as yby baffles or the like.

In accordance with my invention, transmission of sound through the air inlet and outlet openings of the ventilated apparatus housing 1 is substantially blocked by frequencyresponsive air duct structures including resonant cavities. Such cavities are tuned to attenuate selected sound frequencies which predominate in the noise generated by electric apparatus of the alternating current type. For this purpose I mount adjacent each air flow openings 3 and 4 an inwardly-extending unitary assembly of a tubular air passageway and one or more adjacent resonant chambers or cavities, each cavity being in lateral communication with the duct through orifices of predetermined critical dimension in a wall common to the passageway and cavities. Such an integral chambered duct assembly is shown at FIG. 3.

The chambered air duct assembly shown at FIG. 3 comprises a plurality Iof box-like resonating chambers or cavities 29 and 30 having a common top wall 31 and a common bottom wall 32 apertured to provide a separate entrance orifice into each chamber. At FIG. 3, I have shown two chambers 29 of equal dimension and two chambers 30 of equal dimension but different in size from the chambers 29. `Opposite side wall portions 33, 34 of the assembly extend beyond the chambers 29 and 30 in parallel spaced relation and constitute with the apertured bottom wall 32 a tubular air passageway adjacent the apertured sides -of the chambers 29, 30. The resonating chambers 29, 30 and the entrance orifice associated with each constitute sound-attenuating cavities of the Helmholtz type, each tuned to a selected sound frequency to be attenua-ted. The air passageway formed by the common wall 32 and the extended side Walls 33, 34 is positioned adjacent one of the air ow openings 3 or 4, as shown at FIG. 2 and the entire box-like chambered duct assembly is fixed to the housing wall, as by flanges 35. As indicated at FIGS. 1 and 2, a chambered resonating duct assembly of the structure shown at FIG. 3 is aixed to the inner wall of the housing 1 adjacent each of the air flow apertures 3 and 4.

In each of the chambered duct assemblies described, all the resonating cavities are in lateral communication with the tubular air passageway formed by the walls 32, 33 and 34. The wall 32 is common to the passageway and to all the resonating chambers. Chambers tuned to different frequencies are spaced apart along the length of the passageway in the direction of air flow. Each similarly tuned pair of chambers 29 or 30 extends laterally across the air passageway for substantially its full width. It will of course be understood that necessary air flow and resonant cavity dimensioning will determine whether one or several chambers having response to a single frequency are required to span substantially the entire width of the air passageway, Such full transverse extent of each chamber or set of chambers responding to a single frequency is desirable to minimize the amount of sound escaping without attenuation.

It will be further appreciated by those skilled in the art that the chambered duct assemblies may be located either inside or outside the associated air liow opening in the housing 1. In the larger transformer ratings, outside location may be preferable because of restricted interior clearance.

It is known property of a Helmholtz resonator that it acts as a sink for sound waves at its resonant frequency as they pass by the orifice of the resonator. In its most common form the resonator consists -of a tube of length 1 and cross section s connected to a cavity having a volume V. This combination is directly analogous in its sound-attenuating action to a tuned by-pass filter in an electrical circuit.

In a Helmholtz resonator the resonant frequency fo is given by the equation where c is the speed of sound and le is the effective length of the tube equal to 1-l-0.8 \/s. The computation is normally valid if the linear dimensions of the tube are small as compared to the wave length of sound at the resonant frequency )\=c/f0.

Using this relationship the present invention utilizes tubular orifices 29a, 30a of rectangular configuration having areas and lengths such that each associated cavity resonates at a desired sound frequency. The resonant frequencies of the several Helmholtz resonators thus formed may -be set empirically by varying the area of each orifice until the desired frequency is attenuated. Hence a sound wave of frequency of f1 will be severely attenuated as it passes the orifice of the Helmholtz resonator tuned to that frequency.

It has been established that alternating current electric apparatus, such as transformers and other induction apparatus, emit sound primarily at frequencies which are twice the frequency of the fundamental and harmonic frequencies of the applied voltage. These double frequencies I refer to as double harmonics of the fundamental frequency. For example, for an applied voltage at the commercial frequency of 60 cycles per second, the predominant sound frequencies emitted would be 120' c.p.s., 240 c.p.s., 360 c.p.s., 480 c.p.s., etc., with the lower frequencies usually predominating. Hence the present 1invention contemplates a chambered duct arrangement wherein a plurality -of laterally positioned Helmholtz resonators are provided, each cavity or transverse set of cavities being tuned for resonance at a frequency equal to a double harmonic of the applied alternating voltage. In the embodiment shown at FIGS. 1, 2 and 3, four Helmholtz resonators 29 are responsive to sound frequencies of 120 cycles per second, and four resonators 30 are tuned to 240 cycles per second. It is understood that other and higher frequencies may be similarly provided for and that any desired number of double harmonies of a selected fundamental frequency may be attenuated.

As previously pointed out, the air flow openings 3 and 4 in the housing 1 are necessarily quite large when cooling air is to be moved by natural circulation. Such openings have a large periphery and most conveniently are largest in the lateral or width dimension, as shown at FIG. 1. It may thus be necessary to provide a plurality of like Helmholtz resonators tuned to a single frequency in order to span substantially the full width of each opening. Resonators or sets of resonators tuned to other frequencies are arranged along the duct in the direction of air flow, and the one or more resonators at each frequency also span substantially the full width duct. The number of different frequencies (and corresponding sets of resonant chambers) selected for attenuation will depend upon the length of duct available and upon the size of resonator necessary for each frequency to be attenuated.

A chambered duct built in accordance with my invention and installed in a ventilated dry-type transformer in the manner illustrated at FIGS. 1 and 2 was tested in an anechoic chamber, and the results of such test are illustrated graphically at FIG. 4. The chambered duct included Helmholtz resonators tuned to frequencies of c.p.s. and 240 c.p.s. and was built substantially in accordance with the illustration at FIG 3. Tests were made with all the cavity orifices blocked (i.e., no resonant attenuation), and the attenuation at various frequencies is shown at FIG. 4 by the broken line I. Three other tests were `then made using the same resonant cavities but with adjacent air passageway of various heights. The solid-line curves of similar shape each represent the attenuation accomplished by one of these structures with the duct orifices uncovered. It will be observed that in each case the resonating duct structure effected marked attenuation of sound at frequencies of 120 and 240 cycles per second, indicating that egress of sound at these frequencies was effectively reduced.

It is to be understood tht the improved Helmholtz resonator structure embodied in my chambered air duct assembly is most effective when used in cooperation with the other sound attenuation and vibration isolating techniques described above. In this way it is possible to eliminate both direct transmission of sound through the necessarily large air flow openings as well as conduction of sound through the structural parts of the base and housing. It will thus be evident that I have described an integrated mounting and housing structure for air-cooled transformers wherein several features operate cooperatively to confine internally generated sound Within the housing without impeding the flow of cooling air through the housing.

While I have described a preferred embodiment of my invention, other modifications will occur to those skilled in the art. For example, in ducts of large periphery a greater number of resonators or sets of resonators tuned to the same frequency may be arranged about the duct periphery to effect ample attenuation. Moreover, more than one resonator or side-by-side sets of resonators tuned to the same frequency may be arranged serially in the duct to increase sound attenuation. Accordingly, therefore, I wish to have it understood that I intend herein to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an air-cooled electric apparatus of the alternating current type, a ventilated enclosing housing having at least one air flow opening sufficiently large in area to transmit directly therethrough an appreciable proportion of sound generated by apparatus within said housing, a chambered air duct structure mounted adjacent said opening and comprising a tubular air passageway and at least one resonating chamber having a common wall therebetween, said common wall extending transversely across substantially the full width of said air passageway and being provided with a tubular orifice between said chamber and said passageway, said orifice and the associated chamber constituting an acoustic resonator tuned to a selected sound frequency to be attenuated.

2. An electric apparatus according to claim 1 wherein said chambered air duct structure comprises a plurality of like resonating chambers constituting a set all tuned to one selected soundV frequency and extending transversely across substantially the full width of said air passageway.

3. An electric apparatus according to claim 2 wherein said chambered air duct structure comprises a plurality of said sets of like resonant chambers, each said set extending transversely across the associated air passageway and said sets being positioned at spaced-apart locations along the length of said passageway, each said set of chambers being tuned to one of several sound frequencies to be attenuated.

4. An electric apparatus according to claim 1 wherein said housing is mounted substantially in vibration-isolating independence of the enclosed electric apparatus and is provided internally with sound absorbing means interposed between the housing Walls and the enclosed electric apparatus.

5. An electric apparatus according to claim 1 wherein said housing is provided with upper and lower air flow openings and associated chambered air duct structures, said structures being mounted to extend into the housing and to be positioned respectively above and below the enclosed electric apparatus.

References Cited UNITED STATES PATENTS 2,297,269 9/ 1942 Wendt et al. 2,308,886 1/1943 Mason 181-48 2,459,322 l/l949 Johnston 336-59 XR 2,892,507 6/ 1959 Kirkpatrick 336-59 XR 3,210,456 10/1965 Skubal 336-59 XR OTHER REFERENCES Noise Control, periodical, vol. 3, No. 2, March 1957, pp. 37-39 and 86, class 181-33.*4.

ROBERT S. WARD, JR., Prima/'y Examiner.

U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,451,503 June 24, 1969 Thomas J. Twomey It is certified that error appears in the above identified patent and that seid Letters Patent are hereby corrected as shown below:

Column 3, line 41, "3,312,290" should read 3,312,920

Signed and sealed this 9th day of June 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Attesting Officer 

